1. Field of the Invention
The present invention is generally related to the field of semiconductor processing, and, more particularly, to a method for characterizing polish pad lots to eliminate or reduce tool requalification after changing a polishing pad.
2. Description of the Related Art
Chemical mechanical polishing ("CMP") is widely used in semiconductor processing operations as a means of planarizing various process layers, e.g., silicon dioxide, formed above a wafer comprised of a semiconducting material, such as silicon. Chemical mechanical polishing operations typically employ an abrasive slurry distributed in an alkaline or acidic solution to planarize the surface of a process layer through a combination of mechanical and chemical actions.
The continual drive to reduce feature sizes, e.g., channel length, on semiconductor devices has increased the importance of chemical mechanical polishing or planarization in the semiconductor fabrication process. For example, as feature sizes tend to decrease, the depth of field of photolithography equipment tends to shrink, thereby necessitating a very flat surface so that very small dimensions may be accurately patterned on a wafer. Additionally, there has been, and continues to be, a constant drive to increase the productivity of fabrication techniques employed in making modern semiconductor devices. In short, there is a constant drive within the industry to make the same high-quality semiconductor products, but to do it faster, better, and in a less expensive manner.
FIG. 1 is a schematic drawing of one illustrative embodiment of a chemical mechanical polishing tool used in semiconductor processing operations. As depicted therein, the illustrative polishing tool 10 is comprised of a rotatable table 12 on which an illustrative polishing pad 14 is mounted, and a multi-head carrier 16 positioned above the pad 14. The multi-head carrier 16 includes a plurality of rotatable polishing arms 18, each of which includes a carrier head 20. Typically, wafers (not shown) are secured to the carrier heads 20 by the use of vacuum pressure. This is sometimes referred to as the carrier backforce pressure. In use, the table 12 is rotated and an abrasive slurry is dispersed onto the polishing pad 14. Once the slurry has been applied to the polishing pad 14, a downforce is applied to each rotating polishing arm 18 to press its respective wafer against the polishing pad 14. As the wafer is pressed against the polishing pad 14, the surface of the wafer is mechanically and chemically polished. Although the device depicted in FIG. 1 is a multi-head polishing device, similar single-head type machines exist in the industry, and the present invention is not limited to any particular embodiment, form or structure of a tool that may be used to perform chemical mechanical polishing operations.
In general, wafers are polished according to various polishing recipes that may vary, depending upon a variety of factors, e.g., the type of material being polished, the desired rate of removal of the product, etc. During the course of polishing the wafers, the polishing pad 14, which is typically comprised of polyurethane, absorbs or retains various by-products from the polishing operation, such as residual material from the process layer and/or the polishing slurry. The accumulation of such material is known as glazing. Glazing of the polishing pad 14 can affect numerous polishing parameters, e.g., the rate at which material is removed. etc. In general, the polishing pad 14 becomes glazed, the rate of removal of a process layer during polishing operations tends to decrease. This reduction in removal rate may be compensated for by, for example, increasing the duration of the polishing operation, etc.
At some point, a decision is made that the polishing pad 14 is no longer fit for service. At that time, a new polishing pad is placed on the polishing tool 10. Thereafter, the polishing pad 14 is characterized through the use of multiple test wafers. In this characterization process, the test wafers are polished with the new pad and parameters such as the uniformity of the removal across the surface of the wafer, the rate of removal, etc., are determined. Because this process of recharacterizing or requalifying the polishing pad 14 is performed on test wafers, it does not add any value to any finished product, and it can be very time-consuming and expensive.
The present invention is directed to a method of solving, or at least reducing, some or all of the aforementioned problems.